In vehicle braking systems, it is often necessary to limit the hydraulic braking pressure available to the rear brakes. This is done to provide a balance between the front and rear brake forces to ensure adequate vehicle stability. The rear brake pressure proportioning is typically accomplished with a hydraulic valve which controls the rear brake pressure with a fixed proportionality to the front brake pressure. It has been proposed to use the Anti-lock Braking System (ABS) to perform the function of the rear proportioning valve and enhance the performance of the braking system. See British patent 1,431,466 assigned to the assignee of the present invention.